The technical field of this invention is elemental detection and imaging and, in particular, methods and apparatus for the detection of the elemental composition of objects by nuclear interaction analysis. The invention is useful in the detection of contraband concealed within cargo containers, suitcases, parcels or other objects. As used herein, the term "contraband" includes, but is not limited to, explosives, drugs, and alcohol.
During the past ten years, the Federal Aviation Administration (FAA) of the US Department of Transportation has funded considerable research into the prevention of illegal transportation of explosives and drugs. One goal of this research is to create a detection system for airports that will screen passengers' luggage for explosives, as well as other contraband. Once this system is implemented, it is likely to be applied to other inspectional purposes, such as the screening of cargo containers at custom stations, ports, etc., as well.
The probability of the existence of explosives in a piece of luggage at an airport is approximately 1 in 10 million. To avoid lengthy delays at airport security check points, a practical contraband detection system at an airport requires a high detection speed, e.g. 6-8 seconds per piece of luggage and an acceptable false alarm rate. The false alarm rate can be defined as m/n, where n equals the number of the suitcases that the system determines to contain contraband, and m equals the number of suitcases that, upon inspection, do not in fact contain explosives. A false alarm rate of 10-20% or less is preferable.
Similar processing constraints apply to inspection of truck and rail cargo containers at border crossings and other security check points. In both applications, nondestructive detection is required. Damaging effects, such as the activation of the objects under examination, must be minimized. Furthermore, spatial resolution on the order of several centimeters in each dimension is highly desirable.
Various techniques are known for detecting contraband. Metal detectors are routinely used in airports to screen carry-on luggage. While metal detectors are useful in detecting metal weapons they are not imaging systems and most often can not distinguish between weapons and other metallic objects. X-ray imaging systems provide a rudimentary view of objects within a suitcase or container, but suffer from a general inability to image low atomic weight objects (e.g., plastic weapons, explosives and drugs). Moreover, images from conventional X-ray detectors can be stymied by materials, such as metal foils or coatings, that absorb the relatively low energy X-ray radiation and thereby shield the contents from view. Further, X-ray systems determine density or average atomic number but not the existence of explosives, per se.
Ideally, a method of detecting contraband should be capable of distinguishing illegal materials from the typical objects found in luggage or cargo based on distinctive characteristics of the contraband. Thus, elemental analysis of the object undergoing inspection is an important goal for state-of-the-art inspection systems. Typically, explosives have a high nitrogen content, a low carbon-to-oxygen ratio, and high nitrogen and oxygen densities. Drugs, such as cocaine and heroin, have been shown to have high carbon-to-oxygen ratios, high carbon and chlorine contents, and little nitrogen.
Included within nuclear interaction analysis are nuclear emission detection techniques. Nuclear emission detection techniques are based on the realization that characteristic elemental composition data can be obtained from the induced emission of radiation, e.g. gamma-rays, or particles, from the nuclei of the atoms of an object undergoing inspection. According to these techniques a source of radiation, e.g. a particle beam, such as a neutron beam, or a source of hard X-rays or gamma rays, bombards an object under investigation, triggering the nuclei of the object to emit characteristic radiation. In these techniques, referred to generally as "nuclear emission" analyses, different contraband molecules are identified based on their unique nuclear emissions in response to such high energy interrogation. The related term nuclear fluorescence is most commonly used to describe the emission of X-ray radiation by nuclei in response to excitation by X-rays. However, for the purposes of this application nuclear fluorescence will indicate the emission of photons by nuclei in response to excitation by radiation (electromagnetic or particulate).
The emissions are analyzed for characteristic energy profiles that indicate the elemental structures present in the object. Advantageously, nuclear methods can detect the general properties of contraband by identifying and localizing (imaging) the chemical constituents of an object under investigation.
One technique of particular interest at present is known as "fast neutron" analysis. In this approach, fast neutrons (e.g., having energies greater than about 1 MeV, preferably greater than a few MeV) are generated and used to interrogate the object undergoing inspection. The neutrons strike the nuclei of the object and induce gamma ray emissions. Fast neutrons are used because they have high penetration capability and large activation cross-sections with elements of interest, e.g. carbon, nitrogen, and oxygen.
Simple neutron spectroscopy systems merely analyze the spectrum of radiation induced by fast neutrons to detect characteristic emissions. Unfortunately, such data are often insufficient for detection of contraband when the volume of the object is large because the telltale signatures of contraband will be scrambled with the emissions from all the other contents of the object.
Considerable research has been directed towards the development of position-sensitive detection systems for fast neutron and other nuclear emission analyses. Radiographic techniques can be used to construct images. By employing a two-dimensional array of detectors (or a scanning one-dimensional array) a two-dimensional distribution of neutron interaction cross-sections of the object under examination can be obtained. For greater spatial resolution, tomographic approaches can be employed (e.g., using multiple projections from orthogonal arrays of detectors) to construct a three-dimensional image of the emissions. In another approach to acquiring three dimensional data, pulsed neutron beams have been proposed for use with detector arrays, whereby the timing of the detected emissions can provide a degree of depth resolution.
All of the known techniques for nuclear emission detection suffer from one or more deficiencies which make them unattractive for large scale implementation. The spatial resolution of such systems is often compromised by the need to minimize the dose to each object, the limited neutron source strength, and the desire to maintain rapid throughput of objects. Present techniques require strong sources of interrogating radiation. These sources are generally expensive and unreliable. With respect to the requirement of rapid throughput, multiple projection arrays and synchronous timing of such arrays (or pulsed neutron beams) add to the computational overhead and likewise limit throughput. Moreover scanning systems that require moving parts often introduce artifacts that degrade the spatial resolution of the system.
There exists a need for better methods and systems for remote inspection of objects, in general, and for detection of contraband in containers, in particular. A simplified remote inspection system that can provide practical spatial resolution while making efficient use of an interrogating radiation source would satisfy a long-felt need in the art.